Winter precipitation trends in the Mediterranean region for the period 1902 - 2010.

Reds and oranges highlight lands around the Mediterranean that experienced significantly drier winters during 1971-2010 than the comparison period of 1902-2010.

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NOAA study: Human-caused climate change a major factor in more frequent Mediterranean droughts

October 27, 2011

By Katy Human, NOAA Communications

Wintertime droughts are increasingly common in the Mediterranean region,
and human-caused climate change is partly responsible, according to a new
analysis by NOAA scientists and colleagues at the Cooperative Institute for
Research in Environmental Sciences (CIRES). In the last 20 years, 10 of the
driest 12 winters have taken place in the lands surrounding the
Mediterranean Sea.

"The magnitude and frequency of the drying that has occurred is too great
to be explained by natural variability alone," said Martin Hoerling, Ph.D.
of ESRL's Physical Sciences Division in Boulder, CO, lead author
of a paper published online in the Journal of Climate this month. "This is
not encouraging news for a region that already experiences water stress,
because it implies natural variability alone is unlikely to return the
region's climate to normal."

The Mediterranean region accumulates most of its precipitation during the
winter, and Hoerling's team uncovered a pattern of increasing wintertime
dryness that stretched from Gibraltar to the Middle East. Scientists used
observations and climate models to investigate several possible culprits,
including natural variability, a cyclical climate pattern called the North
Atlantic Oscillation (NAO), and climate change caused by greenhouse gases
released into the atmosphere during fossil fuel use and other human
activities.

Climate change from greenhouse gases explained roughly half the increased
dryness of 1902-2010, the team found. This means that other processes, none
specifically identified in the new investigation, also have contributed to
increasing drought frequency in the region.

The team also found agreement between the observed increase in winter
droughts and in the projections of climate models that include known
increases in greenhouse gases. Both observations and model simulations show
a sudden shift to drier conditions in the Mediterranean beginning in the
1970s. The analysis began with the year 1902, the first year of a recorded
rainfall dataset.

In this analysis, sea surface temperature patterns emerged as the primary
reason for the relationship between climate change and Mediterranean
drought. In recent decades, greenhouse-induced climate change has caused
somewhat greater warming of the tropical oceans compared to other ocean
regions. That pattern acts to drive drought-conducive weather patterns
around the Mediterranean. The timing of ocean temperature changes coincides
closely with the timing of increased droughts, the scientists found.

The Mediterranean has long been identified as a "hot spot" for substantial
impact from climate change in the latter decades of this century because of
water scarcity in the region, a rapidly increasing population, and climate
modeling that projects increased risk of drought.

"The question has been whether this projected drying has already begun to
occur in winter, the most important season for water resources," Hoerling
said. "The answer is yes."
Climate is a global phenomenon with global impacts on food prices and water
security, and NOAA researchers are engaged in understanding changes in
climate across many regions of the world. In the Mediterranean, winter
drought has emerged as a new normal that could threaten food security.
Lessons learned from studying climate in that region may also be relevant
for the U.S. West Coast, which has a similar climate to the Mediterranean
region of Europe and North Africa.